Why do women test with faint lines after implantation?

There are so many questions I have about conceiving. But I really want to know, if someone’s period didn’t come in over a week, and they are testing negative or with faint lines. Does this mean their hcg levels just haven’t made it their peak yet? I’m sure if you missed your period you most likely already implanted for it to not come. But should we be able to test positive shortly after implanting?